Heretofore, there has been employed an electrolytically surface-roughening method as one of methods for surface-roughening a support of a PS plate. There have been proposed various methods which employ various alternating current waveforms as a method of controlling the support surface properties. The methods include, for example, a method of employing an alternating waveform with voltage at a positive polarity period greater than voltage at a negative polarity period as disclosed in Japanese Patent Publication Nos. 55-19191 and 56-19280, a method of employing an alternating waveform in which a sine wave alternating current is phase-controlled by a thyristor as disclosed in Japanese Patent Publication No. 57-22036, a method of employing a three-phase alternating current as disclosed in Japanese Patent O.P.I. Publication No. 58-157997, and a method of employing an alternating current in which alternating currents having different cycles are synthesized as disclosed in Japanese Patent O.P.I. Publication No. 58-207374.
However, these methods, when an aluminum alloy support with relatively low purity is used, have a problem in that uniform pits on the support surface can not be obtained. In order to solve the problem, in Japanese Patent O.P.I. Publication No. 1-154797 discloses a method employing an alternating waveform in which an alternating current at a positive polarity period instantly reaches a maximum from zero, followed by a gradual decrease. In this method, in which the current instantly reaches a maximum from zero, however, the following problems are exhibited:
1. A waveform distortion results in non-uniformity in fine structure of the roughened surface of the support.
2. Overvoltage overloads the waveform generator, and is likely to result in mechanical trouble.
3. A presensitized planographic printing plate is prepared in which a light sensitive layer is provided on the support obtained by this method, and when ball point pen ink marks are put on the light sensitive layer in making a printing plate, the ink marks are likely to remain on the support surface without being removed by a developer (causing a ball point pen ink residue problem), resulting in stain occurrence during printing.